Many commercial processes use gas purification and liquid-phase separations. In particular, the chemical and petroleum refining industries require materials that can effectively separate mixtures of hydrocarbon molecules and light gas molecules. Increasingly, these separations are accomplished with molecular sieves, including porous crystalline aluminosilicates (zeolites), that contain pores of molecular dimensions and can therefore exhibit selectivity according to the effective size of a liquid or gas molecule. As a result, zeolites can be used for the separation of mixtures of molecules of varying size.
The effectiveness of a zeolite for separation is determined by the product of its permeability and selectivity. Permeability describes the hydraulic transport resistance of a fluid and is measured by the transmembrane flux per unit pressure difference across a membrane. Selectivity measures the ability of the zeolite to preferentially separate or retain a species from a mixture. Selectivity of a zeolite can be enhanced by tailoring the geometry of the pore (i.e., to alter size exclusion) and by modifying the pore surfaces or acidity of the framework to effect adsorption.
Previous commercial separations of similar hydrocarbon molecules have used either cryogenic distillation or sieving by simple size exclusion. Cryogenic distillation is very energy intensive and pollution producing. Furthermore, it is a non-regenerative process. Separation processes using bulk or membrane zeolites have primarily relied on only size exclusion to achieve physical separation of molecules. Size exclusion alone is inadequate for separations of many commercially important hydrocarbons, such as isoprene, from similar boiling point and similar sized molecules, such as n-pentane, in commercial mixtures. Thus, there is a need to further enhance the selectivity of zeolites by modifying their adsorptive property in order to change the interaction of the diffusing molecules with the internal surfaces of the zeolite structure.
Both bulk and membrane zeolites can be used for molecular separations. Bulk zeolites have the problem of trapping the bulkier molecule in the zeolite pores, thereby generally restricting their use to batch or semi-batch separation processes. Zeolite membranes offer the possibility of continuous separations and zeolite regeneration. However, the synthesis of quality zeolite membranes has been difficult. Because self-supporting zeolite membranes lack durability, most membranes are hydrothermally grown on substrates having pores much larger than the nanoporous zeolite. The as-grown zeolite films tend to have many microdefects or improper pore orientation, which hinder the separation characteristics of the zeolite.
Recently, suitable thin-film membranes that are robust, thermally stable, and have high selectivity with good throughput have been disclosed by Nenoff, et al. in U.S. Pat. No. 6,494,326, which is incorporated herein by reference. These membranes have been fabricated by a combination of growth of zeolite crystallites on a substrate followed by embedding the crystallites in a densified sol-gel product layer, thereby filling in the micropores that otherwise reduce selectivity of the membrane. The zeolite crystals grown inside a membrane tube or on a membrane disk are chosen so their crystal structure allows the passage of gases or molecules of a particular size.
Due to their high acidity and chemical reactivity, many zeolites are not suitable for hydrocarbon separations. Rather, their acidic properties and shape selectivity make zeolites more useful as catalysts for hydrocarbon cracking and isomerization in many refining and petrochemical processes. However, coking from the cracked hydrocarbons is known to deactivate acid sites, resulting in loss of catalytic activity and fouling. Coke generally comprises a mixture of partially decomposed hydrocarbon molecules. See, e.g., U.S. Pat. No. 6,191,331 to Boldingh; U.S. Pat. No. 4,547,613 to Garwood, et al.; C. A. Henriques et al., J. Catalysis 172, 436 (1997); G. D. Pirngruber et al., Microporous and Mesoporous Materials 38, 221 (2000); and H. S. Cerqueira et al., J. Catalysis 196, 149 (2000).
Various post-synthetic coking treatments have been used to “caulk” the microdefects of zeolites with carbonaceous deposits for separations. See, e.g., Y. Yan et al., Journal of Membrane Science 123, 95 (1997). This coking process uses a large aromatic hydrocarbon to fill the microdefects and thereby enhance selectivity, however, at the expense of reduced permeability. Although selectivity was restored, large reductions in permeability have been observed with these “caulked” membranes. It is unlikely that the hydrocarbon molecules enter the zeolite nanopores during the caulking treatment. Therefore, the resulting enhanced selectivity is likely due to size exclusion, rather than selective adsorption.
Therefore, a need remains for the controlled modification of sorptive capacity and pore size of zeolites used for separations. Coking of zeolites provides a means to deactivate the acid sites that cause hydrocarbon cracking, and may provide a means for hydrocarbon separation, if the weak bonding sites required for selective adsorption can be retained. Therefore, the adsorptive properties and selectivity of zeolites may be modified by controlled deposition of carbon into the void volume of the zeolite structure. The present invention provides a method for the controlled carbonization of zeolites. The present invention further provides a method for light hydrocarbon separations using the carbonized zeolite. The method combines the advantages of separations based on variations in molecule size with those based on the differences in molecular adsorption properties. This combination enables effective molecular separations that are not attainable by either size exclusion or differential adsorptivity alone.